Bandwidth in, i.e. downloading the blockchain, is free.  Data transfer out is free for the first 1GB then $0.09 per GB after that.  Which would be expensive running as a full node if someone downloaded the entire blockchain of 172GB, would be ~ $15 every time (not sure if the transfer is measured in bits or bytes which could make a significant difference).  You wouldn't have any transfer out if you were using it privately, i.e. not accepting incoming requests/connections.  Plus, I believe data transfer between servers on the AWS network is free.

You pay for the storage.  I used 200GB of storage which is charged at $0.116 per GB per month, which equates to $23.20 for the month.

The server instance is one of their lower specs and costs $0.0264 per hour, which equates to (roughly) $19 a month.

The beauty of the AWS instances is that you can start and stop them at any time.  You only pay for what you use.  I have used this one for a few days just to download the blockchain and have a play with it.  In 5 1/2 days it's cost me $3.53 for the instance and around $5 for the storage.  My transfer out was 4.6GB, costing 42 cents (I was only accepting incoming connections for a few hours).  It's on a very good network too, so you should get good connectivity.